This invention relates to a process for stabilizing copolymers of ethylene with ethylenically unsaturated carboxylic acids and to stabilized copolymers made by this process.
It is well known to copolymerize ethylene with ethylenically unsaturated carboxylic acids such as, e.g., acrylic and methacrylic acids or with monoesters of unsaturated dicarboxylic acids, such as, e.g., monoethyl maleate or monomethyl maleate. Many such copolymers, sometimes hereinafter referred to as carboxylic acid copolymers. are described in the patent literature, as exemplified by U.S. Pat. Nos. 4,351,931 to Armitage, 3,520,861 to Thompson, and 5,028,674 to Hatch et al.
For safety reasons, it is a common practice in the industry to purge ethylene homopolymers and copolymers with air or nitrogen (collectively referred to hereafter as purge gas) to extract residual ethylene and any other flammable monomers present therein. This renders the polymer product "safe to package" by reducing the amount of these monomers to a level which will not reach a combustible or explosive concentration in the package air space. If purging the polymer with purge gas is not desired, volatile materials can be removed by vacuum extraction. Polymer is normally pelletized by extrusion into rods and underwater cutting, but it is then dried of surface water because the presence of undissolved moisture has always been considered undesirable. Water is known to be the main cause of die buildup from ethylene polymers and it creates undesirable bubbles during melt processing. Drying with heated purge gas results in a very dry material, although the actual amount of water will depend on the degree of hygroscopicity of the polymer, on the temperature and humidity of the purge gas, and on the duration of exposure to purge gas.
Carboxylic acid copolymers are inherently unstable at temperatures greater than about 240.degree. C., used during melt processing, because of side reactions involving the carboxylic groups, especially, anhydride group formation, which may subsequently cause crosslinking that results in an increase of melt viscosity and decrease of melt flow. This has the practical effect of reducing the polymer drawability, as evidenced by edgewave (erratic changing of the width of an extruded melt web) and tearing, increased gel formation in the extruder, and difficulty of purging the extruder clean.
Hoening et al. describe in U.S. Pat. No. 4,594,382 thermally stabilized carboxylic acid copolymers, which contain 5 weight percent or less of a hydrated compound that loses its hydration water between 100.degree. C. and 300.degree. C. A possible alternative of injecting steam into the molten polymer is said in that patent to have produced unsatisfactory results and also to have the further disadvantage of adversely affecting the quality of products made from the copolymers and corroding processing equipment.